Combustion of air/fuel mixture for production of rotating energy is well known in reciprocating internal combustion engines, rotary engines and many others. Beside stoichiometry, what distinguishes one engine from the other mechanically is the system by which air enters the engine, fuel is injected, air/fuel mixture is compressed, the compression ratio is adjusted, the compressed air/fuel mixture is ignited and the combustion force converted to rotating energy.
Matters effecting the suction cycle the most include the time allowed for air to flow in assuming the air throttle is wide open. In a typical reciprocating internal combustion engine for example, it takes 0.03 second at 1,000 RPM (Round-Per-Minute) for air to be sucked inside the cylinder, 0.01 second at 3,000 RPM, 0.006 second at 5,000 RPM and so on. Less time signifies less air-intake, less relative air to fuel ratio, less compression ratio, less combustion force and incomplete burning of fuel that leads to more pollution. The problem worsens at higher elevations as the atmospheric pressure becomes less intense. It is well known that fuel-lean running (use excess air) or turbo charging improves the efficiency if the air/fuel ratio by mass is stoichiometric but turbo charging requires extra sets of turning mechanism that not only contribute to further engine complications but also add more weight and slow down the free flow of exhaust.
On the other hand, the entire notion of suction and expansion within the limitation of a cylinder capacity is incorrect regardless of the engine type. At the expansion cycle, the air mass increases rapidly to a much larger volume, thus, if there is not enough room to take full advantage of the expansion force, a significant portion of it is wasted to the atmosphere. The loud noise that comes out of the exhaust port in any type of engine is the indication of such waste. It is quite clear that the inefficiency of internal combustion engines, regardless of the engine type, may never be solved unless full advantage of the expansion force is achieved.
Another problem is the torque, when the expansion force is converted to rotating energy. The ideal situation of highest torque is to apply the expansion force to the rim of a turning wheel at the direction tangential to the wheel circumference. This milestone is yet to achieve in an engine design.
Another problem is the knock or auto-ignition of compressed air/fuel mixture due to heat and excess compression. Engines are limited in their efficiency by the inability of the fuel to smoothly burn in high compression ratio. A variable compression ratio system is to calibrate compression to meet all variations including heat, atmospheric pressure, fuel type and so forth to optimum performance of the engine.
One other problem is the throw that occurs at exhaust cycle. As a piston and connecting rod move from the bottom death center toward the top dead center, they gather momentum. At the compression cycle because of the trapped air/fuel above the piston, such momentum is neutralized, but at exhaust cycle since there is no resistance over the piston, the centrifugal force particularly at higher RPMs increases the weight of the piston and connecting to such high level that create a major drag against rotation of the crankshaft. Reducing the weight of pistons and connecting rods have diminished the problem to certain extent but since the weight cannot be reduced to zero, the problem will never be solved in the existing engine designs.
When a great portion of the combustion force is not converted to rotating energy, it is converted to heat, heat of such magnitude that can easily burn the engine if not cooled down. Cooling the engine requires a cooling system consisting of double layer engine block, radiator, water pump and so forth that needs additional energy to function, not to mention their contribution to the weight of the engine.
When it comes to suction and compression however, the well-known cylinder, piston connecting rod and crankshaft system is found to be the most reliable and efficient pumping method ever tested.